The AP page specifies its own fonts (basically Helvtica family) which should override whatever defaults are in your browser or operating system. So it's not a font issue unless the whole page is broken not just the spam code.

The AP spam code shows up in my browser (FF 4.0.1 in Windows XP and 7) as a bunch of lines of asterisks and spaces forming letters you can understand by looking but aren't actually letters. It doesn't seem to be an image, as the "curvy" captchas usually are, so it's probably not quite as hard to break. I have problems with some of the image captchas too; some have details that just don't show up right when displayed at normal screen resolution, or are (usually vertically) scrunched when displayed so things get hard to read. But the one here at AP works fine.

To check what's going on, you might try:
* Restart Firefox with no add-ons (used to be called "safe mode"; there's an option to do that in the Firefox 4 Help menu). See if it makes a difference. You might have an add-on (extension, plugin, etc.) that's causing a problem.
* If you have javascript blocked the spam code probably won't work at all and at least will be munged. Make sure javascript is enabled at least for this site.
* View the page source (all the html code behind the page). Right-click anyplace on the page and choose "View Page Source" (a bazillion codes in text mode). Way down the page you should see the collection of asterisks generated when the page loaded for the captcha (it's different whenever you reload). Yes, this a geek trick. Note to David: if I can see the page code anybody can; most pages using the image captchas bury it in obfuscated javascript, at least.

If the problem happens only in Mac Firefox and not other browsers, it might be worth opening a bug report with a link to this site so they can see what's happening.

It's possible that there's something odd about FF on a Mac that isn't in other browsers and operating systems. Perhaps you got a damaged copy -- try downloading again and reinstalling? Working in IE (any version) doesn't say much -- IE pretty much ignores standards.